The present invention relates to a pressure sensing device. More particularly, the invention pertains to a thin shell pressure sensor for measuring in situ pressures in a material mass, such as an arctic ice pack, in which the sensor is embedded.
The accurate measurement of pressure forces within a large material mass is important in a variety of applications. For example, accurate prediction of soil pressures and stresses is important in the design of roads, bridges and other structures. Additionally, measurement of pressure within granular bulk materials, such as grain, is important in designing and monitoring storage facilities for such materials.
Due to the rapid increase in exploration for and production of oil, gas and other minerals in arctic offshore regions, the measurement of pressure within an arctic ice pack is of particular importance. Accurate prediction of such pressures is important in determining environmental design criteria for arctic offshore and coastal structures. Additionally, continuous monitoring of such pressures is required for the proper defense of such structures. See, Templeton, J. S., III, "Island Defense and Monitoring", Proceedings of Technical Seminar on Alaskan Beaufort Sea Gravel Island Design, presented by Exxon Company, U.S.A., Anchorage, Alasks, Oct. 15, 1979, and Houston, Tex., Oct. 18, 1979.
Ice pressure must be determined in situ. Samples removed from the ice pack for subsequent laboratory testing are of limited value since the environmental restraints, once removed, are difficult if not impossible to recreate accurately in a laboratory. Measuring pressures in situ reflects what the ice pack is actually experiencing in terms of pressure. Therefore, the need exists for a reliable pressure sensing device which is capable of accurately sensing in situ pressures while embedded in an arctic ice pack.
One device which has been proposed for the use in measuring pressure in an arctic ice pack is disclosed in Metge, M., et al., "On Recording Stresses in Ice", Proceedings of Third International Symposium on Ice Problems, Aug. 18-21, 1975, Hanover, N.H. Metge recommends the use of a sensor whose stiffness is substantially less than the stiffness of the ice. Metge discloses a sensor having an aluminum plate sandwiched between two layers of an elastomeric material which are in turn sandwiched between two outer aluminum plates. The deformation of the sensor is determined by measuring the change in capacitance between the inner and outer aluminum plates. A correlation is then made between the change in capacitance and the applied pressure.
A second ice pressure sensor is disclosed in Nelson, R. D., et al., "Techniques for Measuring Strees in Sea Ice", University of Alaska, Sea Grant No. 77-1, pages 9-16, January 1977. Contrary to Metge, Nelson recommends use of a sensor several times stiffer than the ice surrounding it. The sensor disclosed by Nelson measures sea ice pressure by sensing the compressive stress in a cylindrical metallic rod which is oriented in the ice pack so as to be loaded axially. Due to its high stiffness, this sensor may be insensitive to small variations in pressure. Also, the sensor may give inaccurate readings because its high stiffness allows it to absorb more of the ice pressure load than the surrounding ice.
Still another device which has been proposed for solving the problem of measuring in situ sea ice pressures is described in U.S. Pat. No. 4,175,445 to Templeton (1979). Templeton discloses a pressure sensing device that is thin in comparison to its width and length. Contrary to both Metge and Nelson, Templeton recommends that the sensor have a stiffness similar to the average anticipated stiffness of the embedding medium. The device has a thin central member which is subjected to reverse curvature bending on application of an external pressure. A plurality of wire resistance strain gauges are used to detect strains in the thin central member due to the bending. The pressure acting on the device may be determined from these strains.
Although the above sensors have provided one solution to the problem of measuring in situ sea ice pressures, the need still exists for a compact, easily deployable sensor which would have the sensitivity of a large ice pressure sensor.